#include <bits/stdc++.h>

using namespace std;

#define int long long

const int N = 210;
int m, s, c;
int a[N], b[N];

bool cmp(int a, int b)
{
    return a > b;
}
signed main()
{
    cin >> m >> s >> c;
    for(int i = 1;i <= c;i ++) cin >> a[i];

    sort(a + 1 , a + 1 + c);
    for(int i = 1;i < c;i ++)
    {
        b[i] = a[i + 1] - a[i] -1;
    }
    sort(b + 1, b + 1 + c, cmp);
    int ret = a[c] - a[1] + 1;
    for(int i = 1;i < m && i < c;i ++)
    {
        ret -= b[i];
    }
    cout << ret << endl;
    return 0;
}